Map Assembly & Silkscreen Outline to L x W Maximum

Unselect this Option and remeasure. 

 

This is only for Surface Mount Silkscreen Outlines. 

The 'origin' of the line is the centerline when unchecked. 

The 'origin' of the line is the inside line edge when checked. 

Silkscreen Line with No Offset.

 

Silkscreen Line with Offset.

 

This is why silkscreen under the package body (especially very small low profile packages) is unacceptable because it can lift the package up just enough to cause solder joint problems. 

Due to the IPC J-STD-001 and the slop tolerance component package positioning that is allowed for assembly acceptability, using the no silkscreen offset can cause soldering problems if the component package moves over the silkscreen during reflow. 

When you Offset the silkscreen outline, it kind of compensates for the package movement (tolerance) during reflow. Especially when you map the silkscreen to the Maximum Body Dimension. 

However, some assembly shops with a higher level of component package movement during reflow, will use the Offset Margin Option to move the silkscreen further away from the package body. 

It depends on the assembly shop quality and accuracy. Some shops use older equipment and require robust tolerances. While some assembly shops use new equipment with very high placement accuracy levels. 

Talk to your assembly shop and ask them what there placement tolerances are.